Method for producing retardation film

ABSTRACT

A method for producing a retardation film comprising the steps of: (a) uniaxially stretching an original film for producing retardation film in one direction at either a temperature T 1  or T 2 ; and then (b) uniaxially stretching the film stretched in the step (a) in a direction perpendicular to the above-mentioned direction of stretching at a temperature T 2  or T 1  different from the above-mentioned temperature, in which the original film for producing retardation film has a characteristic that a phase of linearly polarized light entering vertically into the film plane and having an oscillating surface of an electric vector in an X-Z plane against linearly polarized light entering vertically into the film plane and having an oscillating surface of an electric vector in a Y-Z plane lags by uniaxially stretching in the direction of the X axis at a temperature T 1 , and leads by uniaxially stretching in the direction of the X axis at a temperature T 2  different from the above-mentioned temperature T 1 , in which the X axis is an uniaxially stretching direction, the Y axis is a direction perpendicular to the uniaxially stretching direction in the film plane, and the Z axis is a direction of a thickness of the film.

This application is a Continuation of copending application Ser. No.12/371,317, filed on Feb. 13, 2009, which claims priority under 35U.S.C. §119(e) to U.S. Provisional Application No. 61/080,945, filed onJul. 15, 2008, and under 35 U.S.C. §119(a) to Application No.2008-033782, filed in JAPAN on Feb. 14, 2008, all of which are herebyexpressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to a method for producing a retardationfilm. More particularly, the present invention relates to a method forproducing a retardation film suitable for birefringence compensation ofa liquid crystal display device.

BACKGROUND OF THE ART

To decrease dependence of a color tone in a liquid crystal displaydevice on an angle, a retardation film which satisfies a relation of0.92≦R₄₀/Re≦1.08, in which Re is a retardation value at an incidentangle of 0 degrees, and R₄₀ is a retardation value at an incident angleof 40 degrees, or a retardation film which satisfies a relation ofn_(x)>n_(z)>n_(y), in which n_(x) is a refractive index in a slow axisdirection in a plane, n_(y) is a refractive index in a direction at aright angle to the slow axis direction in the plane, and n_(z) is arefractive index in a thickness direction, have been proposed.

For example, Patent Document 1 discloses that a first anisotropic filmis obtained by uniaxially stretching a polycarbonate resin film, asecond anisotropic film is obtained by uniaxially stretching apolystyrene resin film on the other hand, and then a retardation filmwhich satisfies a relation of n_(x)>n_(z)>n_(y) is obtained byoverlapping the first anisotropic film and the second anisotropic filmso that the directions of stretching of the films are at right angles toeach other.

Patent Document 2 discloses that a first anisotropic film is obtained byuniaxially stretching a polycarbonate resin film, a second anisotropicfilm is obtained by uniaxially stretching a polystyrene resin film onthe other hand, and then a retardation film which satisfies a relationof (Re−Re₄₀)/Re≦0.07 is obtained by overlapping the first anisotropicfilm and the second anisotropic film so that the directions ofstretching are at right angle to each other.

In these producing methods described in the patent document 1 or patentdocument 2, an accurate axis match is required when the films are stucktogether.

Patent Document 3 discloses that a laminated body is formed by bonding ashrinkable film to one side or both sides of a resin film before theresin film is stretched, and stretching and heating of the laminatedbody so as to give a contractive force in a direction perpendicular to astretching direction of the above-mentioned resin film results inobtaining a retardation film which satisfies a relation of0<(n_(x)−n_(z))/(n_(x)−n_(y))<1.

In the producing method described in the patent document 3, thecontractive force should be controlled accurately.

Patent Document 4 discloses that a rod is obtained by extrusion moldingof a melted polycarbonate resin, a disc is obtained by slicing up therod in round, a rectangular film is obtained by cutting the disc, andthen a retardation film which satisfies a relation of 0.92≦Re₄₀/Re≦1.08is obtained by uniaxially stretching the rectangular film.

However, it is difficult to produce the retardation film having a largearea by the producing method described in the Patent Document 4.

PRIOR ART DOCUMENTS

-   [Patent Document 1] Japanese Published Unexamined Patent Application    No. H03-24502-   [Patent Document 2] Japanese Published Unexamined Patent Application    No. H03-141303-   [Patent Document 3] Japanese Published Unexamined Patent Application    No. H05-157911-   [Patent Document 4] Japanese Published Unexamined Patent Application    No. H02-160204

SUMMARY OF THE INVENTION Problems to be Resolved by the Invention

An object of the present invention is to provide a method for producinga retardation film comprising a laminate of a plurality of layers sothat the molecular orientation axes of the layers cross each other at aright angle, with good productivity without needing the step of stickingtogether to match the axis.

In addition, an object of the present invention is to provide a simpleand high-accuracy method for producing a retardation film having a largearea which satisfies a relation of n_(x)>n_(z)>n_(y); or a retardationfilm having a large area which satisfies a relation of 0.92≦R₄₀/Re≦1.08.

Means for Solving the Problems

As the result of studies by the present inventor to achieve theabove-mentioned objects, it was found that a retardation film having alarge area which satisfies a relation of n_(x)>n_(z)>n_(y), in whichn_(x) is a refractive index in a slow axis direction in a plane of thefilm, n_(y) is a refractive index in a direction at a right angle to theslow axis direction in the plane, and n_(z) is a refractive index in athickness direction, or satisfies a relation of 0.92≦R₄₀/Re≦1.08, inwhich Re is a retardation value at an incident angle of 0 degrees, andR₄₀ is the retardation value at an incident angle of 40 degrees, can behighly accurate and easily manufactured by (a) uniaxially stretching anoriginal film for producing retardation film in a direction at either atemperature T1 or T2, and then, (b) uniaxially stretching the filmstretched in the step (a) in a direction perpendicular to the directionof the stretching in the step (a) at a different temperature T2 or T1from the temperature in the step (a), in which the original film forproducing retardation film has a characteristic that a phase of linearlypolarized light entering vertically into a film plane and having anoscillating surface of an electric vector in an X-Z plane against thatof linearly polarized light entering vertically into the film plane andhaving an oscillating surface of an electric vector in a Y-Z plane lagsby uniaxially stretching in a direction of the X axis at a temperatureT1, and leads by uniaxially stretching in a direction of the X axis at atemperature T2 different from the above-mentioned temperature T1, inwhich the X axis is an uniaxially stretching direction, the Y axis is adirection perpendicular to the uniaxially stretching direction in thefilm plane, and the Z axis is a direction of a thickness of the film.

The present invention has been completed based on these findings andfurther studies.

That is, the present invention includes the following modes.

(1) A method for producing a retardation film which satisfies a relationof n_(x)>n_(z)>n_(y), in which n_(x) is a refractive index in a slowaxis direction in a plane of the film, n_(y) is a refractive index in adirection perpendicular to the slow axis direction in the plane, andn_(z) is a refractive index in a thickness direction, comprising stepsof: (a) uniaxially stretching an original film for producing retardationfilm in one direction at either a temperature T1 or T2; and then (b)uniaxially stretching the film stretched in the step (a) in a directionperpendicular to the above-mentioned direction of stretching in the step(a) at a temperature T2 or T1 different from the above-mentionedtemperature in the step (a), in which the original film for producingretardation film has a characteristic that a phase of linearly polarizedlight entering vertically into the film plane and having an oscillatingsurface of an electric vector in an X-Z plane against that of linearlypolarized light entering vertically into the film plane and having anoscillating surface of an electric vector in a Y-Z plane lags byuniaxially stretching in a direction of the X axis at a temperature T1,and leads by uniaxially stretching in a direction of the X axis at atemperature T2 different from the above-mentioned temperature T1, inwhich the X axis is an uniaxially stretching direction, the Y axis is adirection perpendicular to the uniaxially stretching direction in thefilm plane, and the Z axis is a direction of a thickness of the film.

(2) A method for producing a retardation film which satisfies a relationof 0.92≦R₄₀/Re≦1.08, in which Re is a retardation value at an incidentangle of 0 degrees, and R₄₀ is a retardation value at an incident angleof 40 degrees, comprising steps of: (a) uniaxially stretching anoriginal film for producing retardation film in one direction at eithera temperature T1 or T2; and then (b) uniaxially stretching the filmstretched in the step (a) in a direction perpendicular to theabove-mentioned direction of stretching in the step (a) at a temperatureT2 or T1 different from the above-mentioned temperature in the step (a),in which the original film for producing retardation film has acharacteristic that a phase of linearly polarized light enteringvertically into the film plane and having an oscillating surface of anelectric vector in an X-Z plane against that of linearly polarized lightentering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane lags by uniaxiallystretching in a direction of the X axis at a temperature T1, and leadsby uniaxially stretching in a direction of the X axis at a temperatureT2 different from the above-mentioned temperature T1, in which the Xaxis is a uniaxially stretching direction, the Y axis is a directionperpendicular to the uniaxially stretching direction in the film plane,and the Z axis is a direction of a thickness of the film.

(3) An original film for producing retardation film having acharacteristic that a phase of linearly polarized light enteringvertically into the film plane and having an oscillating surface of anelectric vector in an X-Z plane against that of linearly polarized lightentering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane lags by uniaxiallystretching in a direction of X axis at a temperature T1, and leads byuniaxially stretching in a direction of X axis at a temperature T2different from the above-mentioned temperature T1, in which X axis is anuniaxially stretching direction, Y axis is a direction perpendicular tothe uniaxially stretching direction in the film plane, and Z axis is adirection of a thickness of the film.

(4) The original film for producing retardation film according to theabove (3), in which the original film is a laminate of a layer composedof a thermoplastic resin A having a positive intrinsic birefringence anda layer composed of a thermoplastic resin B having a negative intrinsicbirefringence.

(5) The original film for producing retardation film according to theabove (3) or (4), in which an absolute value of difference between adeflection temperature under load Ts_(A) of the thermoplastic resin Aand a deflection temperature under load Ts_(B) of the thermoplasticresin B is 5° C. or more.

(6) The original film for producing retardation film according to anyone of the above (3) to (5), in which a rupture elongation of thethermoplastic resin A at the temperature Ts_(B) and a rupture elongationof the thermoplastic resin B at the temperature Ts_(A) are both 50% ormore.

(7) The original film for producing retardation film according to anyone of the above (3) to (6), in which the thermoplastic resin A is apolycarbonate resins and the thermoplastic resin B is a polystyreneresins.

(8) The original film for producing retardation film according to anyone of the above (3) to (7), in which a ratio of sum total thickness oflayers composed of the thermoplastic resin A and sum total thickness oflayers composed of the thermoplastic resin B is 1:5 to 1:10.

Advantages of the Invention

In a method for producing a retardation film according to the presentinvention, a retardation film with a large area which satisfies arelation of n_(x)>n_(z)>n_(y) or a relation of 0.92≦R₄₀/Re≦1.08 can beaccurately and easily manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a temperature dependence of retardations of layer A, layerB, and a laminate of layer A and layer B.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the method for producing a retardation filmwhich satisfies a relation of n_(x)>n_(z)>n_(y), in which n_(x) is arefractive index in a slow axis direction in a plane, n_(y) is arefractive index in a direction perpendicular to the slow axis directionin the plane, and n_(z) is a refractive index in a thickness directionof the film and/or satisfies a relation of 0.92≦R₄₀/Re≦1.08, in which Reis a retardation value at an incident angle of 0 degrees, and R₄₀ is aretardation value at an incident angle of 40 degrees comprises the stepsof: (a) uniaxially stretching an original film for producing retardationfilm described later in a direction at either a temperature T1 or T2;and then (b) uniaxially stretching the film stretched in the step (a) ina direction perpendicular to the above-mentioned direction of stretchingin the step (a) at a temperature T2 or T1 different from theabove-mentioned temperature in the step (a).

(An Original Film for Producing Retardation Film)

An original film for producing retardation film in the present inventionhas a characteristic that a phase of linearly polarized light enteringvertically into the film plane and having an oscillating surface of anelectric vector in an X-Z plane against that of linearly polarized lightentering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane lags by uniaxiallystretching in a direction of X axis at a temperature T1, and leads byuniaxially stretching in a direction of X axis at a temperature T2different from the above-mentioned temperature T1, in which X axis is anuniaxially stretching direction, Y axis is a direction perpendicular tothe uniaxially stretching direction in the film plane, and Z axis is adirection of a thickness of the film.

In a film that a slow axis appears in the X axis by the uniaxialstretching, a phase of linearly polarized light having an oscillatingsurface of an electric vector in an X-Z plane lags against that oflinearly polarized light having an oscillating surface of an electricvector in a Y-Z plane. Oppositely, in a film that a fast axis appears inthe X axis by the uniaxial stretching, a phase of linearly polarizedlight having an oscillating surface of an electric vector in an X-Zplane leads against that of linearly polarized light having anoscillating surface of an electric vector in a Y-Z plane.

The original film for producing the retardation film in the presentinvention is a film which has a dependence of appearance of the slowaxis or the fast axis on a stretching temperature.

Such a film having the temperature dependence of the phase differenceappearance can be obtained by laminating a layer composed of athermoplastic resin A that has a positive intrinsic birefringence and alayer composed of a thermoplastic resin B that has a negative intrinsicbirefringence while adjusting a relationship of a photoelasticcoefficient of the thermoplastic resins and a ratio of thicknesses ofthe each resin layer, and the like.

In this specification, a positive intrinsic birefringence means that arefractive index in a stretching direction is larger than a refractiveindex in a direction perpendicular to the stretching direction, and anegative intrinsic birefringence means that a refractive index in astretching direction is smaller than a refractive index in a directionperpendicular to the stretching direction. The intrinsic birefringencemay be calculated from the permittivity distribution.

A thermoplastic resin A having a positive intrinsic birefringenceincludes: olefin resins such as polyethylene, polypropylene and thelike; polyester resins such as polyethylene terephthalate, polybutyleneterephthalate and the like; polyarylene sulfide resins such aspolyphenylene sulfide and the like; polyvinyl alcohol resins,polycarbonate resins, polyarylate resins, cellulose ester resins,polyether sulfone resins, polysulfone resins, polyallyl sulfone resins,polyvinyl chloride resins, norbornene resins, rod-like liquidcrystalline polymers, and the like. The resin may be used as single orin combination of two or more. In the present invention, among these,polycarbonate resin is preferable according to the viewpoint of anappearance of a phase difference, a stretching property at a lowtemperature, and an adhesive quality with another layer.

A deflection temperature under load Ts_(A) of the above-mentionedthermoplastic resin A is preferably 80° C. or more, more preferably 110°C. or more, and particularly preferably 120° C. or more. When thedeflection temperature under load is lower than the above-mentionedlower limit value, the orientation easily relaxes.

A thermoplastic resin B having a negative intrinsic birefringenceincludes polystyrene resins such as homopolymer of styrene or styrenederivative, or copolymers thereof with other monomer; polyacrylonitrileresins, polymethyl methacrylate resins, or multicomponent copolymersthereof and the like. The resin may be used as single or in combinationof two or more. As the other monomers which is contained in thepolystyrene resins, acrylonitrile, maleic anhydride, methylmethacrylate, and butadiene are preferably mentioned. In the presentinvention, among these, polystyrene resins is preferable in theviewpoint of an excellent appearance of a phase difference, and moreoverthe copolymer of styrene or styrene derivative and maleic anhydride isespecially preferable in the point of excellent thermal resistance.

A deflection temperature under load Ts_(B) of the above-mentionedthermoplastic resin B is preferably 80° C. or more, more preferably 110°C. or more, and particularly preferably 120° C. or more. When thedeflection temperature under load is lower than the above-mentionedlower limit value, an orientation easily relaxes.

An absolute value of a difference between the deflection temperatureunder load Ts_(A) of a thermoplastic resin A and the deflectiontemperature under load Ts_(B) of a thermoplastic resin B is preferably5° C. or more, more preferably 5 to 40° C., and especially preferably 8to 20° C. When the difference between the deflection temperatures underload is too small, a temperature dependence of an appearance of a phasedifference decreases. When the difference between the deflectiontemperatures under load is too large, it becomes difficult to stretch athermoplastic resin having a high deflection temperature under load, andthe planarity of a retardation film is easy to decrease. Theabove-mentioned deflection temperature under load Ts_(A) of athermoplastic resin A is preferably higher than the deflectiontemperature under load Ts_(B) of a thermoplastic resin B.

Each of a rupture elongation of a thermoplastic resin A at a temperatureof Ts_(B) and a rupture elongation of a thermoplastic resin B at atemperature of Ts_(A) is preferably 50% or more, and more preferably 80%or more. A thermoplastic resin having the rupture elongation in thisrange can stably provide the retardation film by stretching. The ruptureelongation is measured using a test piece of type 1B described in JIS(Japanese Industrial Standard) K 7127 at the drawing speed of 100mm/minute.

Compounding agents may be added to thermoplastic resin A and/orthermoplastic resin B, if a total light transmittance in 1 mm thicknessmay be maintained to 80% or more. The added compounding agent is notespecially limited. Examples of the compounding agent includelubricants; lamellar crystal compounds; inorganic particulates;stabilizers such as antioxidant, thermal stabilizers, opticalstabilizers, weathering stabilizers, ultraviolet absorbers, andnear-infrared radiation absorbents; plasticizer; colorant such as dyesand pigments; antistatic agent; and the like. The amount of thecompounding agent may be accordingly decided within a range where theobject of the present invention is not ruined. In particular, lubricantor ultraviolet absorber may be preferably added to improve a flexibilityand a weather resistance.

The lubricant includes inorganic particles such as silica dioxide,titanium dioxide, magnesium oxide, calcium carbonate, magnesiumcarbonate, barium sulphate, and strontium sulphate; organic particlessuch as polymethyl acrylate, polymethyl methacrylate, polyacrylonitrile,polystyrene, cellulose acetates, and cellulose acetate propionates. Inthe present invention, the organic particle is preferable as thelubricant.

The ultraviolet absorber includes oxybenzophenone compounds,benzotriazol compounds, salicylate ester compounds, benzophenoneultraviolet absorbers, benzotriazol ultraviolet absorbers, acrylonitrileultraviolet absorbers, triazine compounds, nickel complex saltcompounds, and inorganic fine particles. The preferred ultravioletabsorber includes2,2′-methylene-bis(4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol),2-(2′-hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chlorobenzotriazol,2,4-di-tert-butyl-6-(5-chlorobenzotriazol-2-yl)phenol,2,2′-dihydroxy-4,4′-dimethoxybenzophenone, and2,2′,4,4′-tetrahydroxy-benzophenone. The especially preferredultraviolet absorber includes2,2′-methylene-bis(4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol).

Each of a layer composed of a thermoplastic resin A and a layer composedof a thermoplastic resin B may have one layer or more than two layers. Aphase difference is a value obtained by multiplying a thickness d by adifference (=n_(x)−n_(y)) between a refractive index n_(x) in adirection of an X axis that is a stretching direction and a refractiveindex n_(y) in a direction of a Y axis perpendicular to the stretchingdirection. A phase difference of a laminated body of a layer (layer A)composed of a thermoplastic resin A and a layer (layer B) composed of athermoplastic resin B is a combine of a phase difference of the layer Aand a phase difference of the layer B. In order that signs of a phasedifference of the laminated body composed of the layer A and the layer Bmay invert by stretching at a high temperature T_(H) and a lowtemperature T_(L), it is preferable to adjust the thickness of the bothresin layers so as to be an absolute value of a phase difference whichappears in a resin having a high deflection temperature under loadsmaller than the absolute value of the phase difference which appears ina resin having a low deflection temperature under load by stretching atlow temperature T_(L), and so as to be an absolute value of the phasedifference which appears in the resin having the low deflectiontemperature under load smaller than an absolute value of the phasedifference which appears in the resin having the high deflectiontemperature under load by stretching at high temperature T_(H). In thisway, adjustments of a difference between refractive index n_(X) in adirection of X axis and refractive index n_(Y) in a direction of Y axiswhich appears in each of layer A and layer B by uniaxial stretching; thetotal thickness of layer(s) A; and the total thickness of layer(s) B cangive a film having a characteristic that a phase of linearly polarizedlight entering vertically into a film plane and having an oscillatingsurface of an electric vector in an X-Z plane against a phase oflinearly polarized light entering vertically into the film plane andhaving an oscillating surface of an electric vector in a Y-Z plane lagsby uniaxially stretching in a direction of the X axis at a temperatureT1, and leads by uniaxially stretching in the direction of the X axis ata temperature T2 different from the above-mentioned temperature T1.Here, the temperature T1 is a temperature either T_(H) or T_(L), and thetemperature T2 is a temperature either T_(H) or T_(L) different from T1.

FIG. 1 shows a temperature dependence of phase difference of layer Awhich is a layer composed of a thermoplastic resin A having a highdeflection temperature under load or layer B which is a layer of athermoplastic resin B with a low deflection temperature under load in anoriginal film for producing retardation film in the present inventionwhen layer A or layer B is respectively stretched; and a temperaturedependence of phase difference of the original film for producingretardation film in the present invention when the original film (layerA+layer B) is stretched. By stretching at the temperature Tb, since aphase difference of a minus that appears by layer B is bigger than aphase difference of a plus that appears by layer A, a phase differenceof a minus Δappears by (layer A+layer B). On the other hand, bystretching at the temperature Ta, since a phase difference of a minusthat appears by layer B is smaller than a phase difference of a plusthat appears by layer A, a phase difference of a plus Δappears by (layerA+layer B).

For example, when layer A is a polycarbonate resin, and layer B is acopolymer of styrene and maleic anhydride, a ratio of a sum totalthickness of layer(s) A and a sum total thickness of layer(s) B ispreferably 1:5 to 1:15, and more preferably 1:5 to 1:10. If layer Abecomes thick too much or layer B becomes thick too much, a temperaturedependence of a appearance of a phase difference decreases.

A total thickness of an original film for producing retardation film inthe present invention is preferably 10 to 500 μm, more preferably 20 to200 μm, and especially preferably 30 to 150 μm. When the above-mentionedtotal thickness is thinner than 10 μm, it is difficult to obtain anenough phase difference and mechanical strength of the film weakens.When the above-mentioned total thickness is thicker than 500 μm,flexibility deteriorates and it might interfere with handling.

The thickness of layer A and layer B is decided by measuring a totalthickness of the film using a contact thickness gauge in themarketplace; cutting a portion where the thickness is measured;observing the cutting surface with a photon microscope to measure aratio of a thickness of an each layers; and calculating the thickness oflayer A and layer B from the ratio. The above-mentioned operations weredone in a direction of MD and a direction of TD of the film at constantintervals, and a mean value of the thickness and the data spread of thethickness were decided.

Here, the data spread of thickness is calculated by the followingexpressions:A data spread of a thickness (μm)=Large one of T _(ave) −T _(min) or T_(max) −T _(ave)

in which T_(max) represents the maximum value in the measured thicknessT, T_(min) represents the minimum value in the measured thickness T, andT_(ave) represents an arithmetic mean value of thickness T measured bythe above-mentioned measurements.

When the data spread of the thickness of layer A and layer B is 1 μm orless in the entire plane, the variability of a color tone becomes smalland a change of a color tone after long-term using becomes uniform.

The following steps are performed to adjust the data spread of thethickness of layer A and layer B to 1 μm or less in the entire plane:(1) a polymer filter having a mesh spacing of 20 μm or less is attachedin an extruding machine; (2) a gear pump is rotated by 5 rpm or more;(3) an enclosure means is attached to surroundings of a die; (4) anairgap is set to 200 mm or less; (5) an edge pinning is performed when afilm is casted to a cooling roll; and (6) a two axis extruding machineor a single axis extruding machine which has a double flight type screwis used as the extruding machine.

An original film for producing retardation film in the present inventionmay have a layer other than layer A and layer B. Examples of the otherlayer include a bonding layer to bond layer A and layer B, a mat layerto improve slipperiness of the film, a hard coat layer such as animpact-resistant polymethacrylate resin layer, and the like, anantireflection layer, and an antifouling layer, and the like.

The original film for producing the retardation film in the presentinvention is preferably 85% or more in a total light transmittance. Whenthe total light transmittance is less than 85%, the film tends to beunsuitable to an optical material. The above-mentioned lighttransmittance is measured by using the spectrophotometer (manufacturedby JASCO Corporation; Ultraviolet Visible Near-infraredSpectrophotometer “V-570”) in accordance with JIS K 0115.

A haze of the original film for producing the retardation film in thepresent invention is preferably 5% or less, more preferably 3% or less,and especially preferably 1% or less. When the haze is high, a sharpnessof a display image tends to decrease. Here, the haze is a mean value ofturbidities measured at five places by using “Turbidimeter NDH-300A”manufactured by Nippon Denshoku Industries Co., Ltd. in accordance withJIS K 7361-1997.

The original film for producing the retardation film in the presentinvention is preferably 5 or less, and more preferably 3 or less in ΔYI.When the ΔYI is within the above-mentioned range, a visibility improvessince there is not coloring. ΔYI is measured by using Spectro colordifference meter “SE2000” manufactured by the Nippon Denshoku IndustriesCo., Ltd. in accordance with ASTM E313. The Similar measurement iscarried out five times, and the ΔYI is obtained as an arithmetic meanvalue of the measurements.

The original film for producing the retardation film in the presentinvention is preferably H or harder in the JIS pencil hardness. This JISpencil hardness may be adjusted by changing a kind of a resin andchanging the thickness of the resin layer, and the like. A surface of afilm is scratched with a pencil having various hardness inclined to 45degrees and pushed on by a load weight of 500 gram-weight, in accordancewith JIS K 5600-5-4, sequentially from the pencil having low hardness,and the JIS pencil hardness means a hardness of the first pencil bywhich the scar is applied to the film.

A surface of the outside of the original film for producing theretardation film in the present invention is preferably smooth and haspreferably substantially neither a linear concave portion nor a linearconvex portion (so-called die line) that is parallel in the direction ofMD and is irregularly formed. Here, “be smooth and have substantiallyneither a linear concave portion nor a linear convex portion that isirregularly formed” means that a depth of the linear concave portion isless than 50 nm or a width of the linear concave portion is bigger than500 nm and a height of the linear convex portion is less than 50 nm or awidth of the linear convex portion is bigger than 500 nm, even if thelinear concave portion or the linear convex portion is formed.Preferably, the depth of the linear concave portion is less than 30 nmor the width of the linear concave portion is bigger than 700 nm and theheight of the linear convex portion is less than 30 nm or the width ofthe linear convex portion is bigger than 700 nm. This formation mayprevent a light interference, an optical leakage and the like caused bythe refraction of light at the linear concave portion or the linearconvex portion, which may result in improving an optical performance.“be irregularly formed” means that the linear concave portion and thelinear convex portion are formed at the unintended position, with theunintended size, the unintended shape and the like.

The above-mentioned depth of the linear concave portion, the height ofthe linear convex portion, and the width of those may be measured by themethod of the description as follows. Light is irradiated to theoriginal film for producing the retardation film, transmitted light isprojected onto a screen, then a part where stripes of light and shadeexist that appears on the screen (in this part, the depth of a linearconcave portion and the height of a linear convex portion are big) iscut out in square of 30 mm×30 mm. The surface of the film section cutout is observed by using a three-dimensional surface structure analyzingmicroscope (view area of 5 mm×7 mm), this observation result isconverted into a three dimension image, and a cross-section profile isobtained from the three dimensional image. Here, the cross-sectionprofile is obtained at intervals of 1 mm in a view area.

An average line is drawn at this cross-section profile, then length fromthis average line to the bottom of a linear concave portion is taken asa depth of linear concave portion, moreover length from this averageline to a top of the linear convex portion is taken as a height of alinear convex portion. The distance between intersections of the averageline and the profile line is taken as width. Each of the maximum valueis obtained from the measured depth of the linear concave portion andthe measured height of the linear convex portion. And each of the widthof the linear concave portion or the linear convex portion thatindicates the maximum value is obtained. The maximum value of the depthof the above-mentioned linear concave portion is taken as a depth of thelinear concave portion of the film, and the maximum value of the heightof the above-mentioned linear convex portion is taken as a height of thelinear convex portion of the film, and the width of a linear concaveportion that indicates the maximum depth value is taken as the width ofa linear concave portion of the film, and the width of a linear convexportion that indicates the maximum height value is taken as the width ofa linear convex portion of the film.

The original film for producing the retardation film in the presentinvention is not especially limited by the producing method thereof.Mentioned as the producing method is a well-known method including acoextrusion molding method such as a coextrusion T-die method, acoextrusion inflation method, and a coextrusion lamination method andthe like; a film lamination molding method such as a dry lamination andthe like; and a coating molding method such as an application of a resinsolution on a resin film; and the like. In particular, the coextrusionmolding method is preferable from the viewpoint of producing efficiencyand preventing volatile contents such as solvents from remaining in thefilm. The coextrusion T-die method is preferable in the coextrusionmolding method. There are a feed block method and a multi manifoldmethod as the coextrusion T-die method. The multi manifold method isespecially preferable from the viewpoint of decreasing the variabilityof the thickness of layer A.

When the coextrusion T-die method is adopted as a method of obtainingthe multilayer film, a temperature of melted resin material in extrudingmachine equipped with T-die is preferably a temperature that is higherthan a glass transition temperature (Tg) of the thermoplastic resin usedas each resin material by 80 to 180° C., and more preferably atemperature that is higher than the glass transition temperature by 100to 150° C. When the temperature of melted resin material in theextruding machine is excessive low, a flowability of the resin materialmight be insufficient, oppositely when the temperature of melted resinmaterial is excessive high, the resin might be deteriorated.

The temperature of extrusion may be properly selected according to thethermoplastic resin used. A temperature of a resin slot is preferably Tgto (Tg+100)° C., a temperature of an exit of the extruding machine ispreferably (Tg+50)° C. to (Tg+170)° C., and a temperature of a die ispreferably (Tg+50)° C. to (Tg+170)° C., in which these temperatures aremeasured in the extruding machine. Here, Tg means a glass transitiontemperature of a thermoplastic resin A used as a resin material.

In the extrusion molding method, a sheeted melted resin materialextruded from a opening of the die is pinned on a cooling drum. A methodof pinning a melted resin material on a cooling drum is not especiallylimited, and examples of the method include an air knife method, avacuum box method, and a electrostatic pinning method, and the like.

The number of cooling drums is usually two or more, though the number isnot particularly limited. Moreover, examples of a method of arrangingthe cooling drums include a straight line type, a Z type, and a L type,though the method is not particularly limited. Moreover, the method ofpassing the melted resin extruded from the opening of the die betweenthe cooling drums is not particularly limited.

In the present invention, the adhesion to the cooling drum of theextruded sheeted resin material changes depending on the temperature ofthe cooling drum. The adhesion improves when a temperature of thecooling drum is raised. However, trouble that the sheeted resin materialcoils around the drum without peeling off from the cooling drum mightoccur when the temperature of the cooling drum is raised too much.Therefore, a temperature of the cooling drum is preferably no more than(Tg+30)° C., and more preferably (Tg−5)° C. to (Tg−45)° C., in which Tgis a glass transition temperature of a thermoplastic resin A extrudedfrom the die. The trouble such as slipping and wounds may be preventedby doing so.

Moreover, it is preferable to reduce a content of a residual solvent inthe film. Examples of the method for reducing the content include (1) amethod of reducing residual solvent in thermoplastic resin as rawmaterial; (2) a method of preliminary drying resin material before filmis molded; and the like. The preliminary drying is done by using the hotair dryer and the like for example after the resin material is moldedinto the form such as pellets and the like. The drying temperature ispreferable 100° C. or more, and the drying time is preferable 2 hours ormore. By doing the preliminary drying, the residual solvent in the filmmay be decreased, and the extruded sheeted resin material may beprevented from foaming.

The step of preheating the original film for producing the retardationfilm (preheating process) may be performed before the original film forproducing the retardation film is stretched. As a means to heat theoriginal film for producing the retardation film, a oven type heatingdevice, a radiation heating device, soaking the film in the liquid, andthe like are mentioned. In these, the oven type heating device isespecially preferable. A heating temperature in the preheating processis usually (the stretching temperature−40)° C. to (the stretchingtemperature+20)° C., and preferably (the stretching temperature−30)° C.to (the stretching temperature+15)° C. The stretching temperature meansthe preset temperature of the heating unit.

(First Stretching Process)

In the present invention, the above-mentioned original film forproducing retardation film is uniaxially stretched at either atemperature T1 or T2, first. When stretching at temperature T1, a phaseof linearly polarized light entering vertically into the film plane andhaving an oscillating surface of an electric vector in an X-Z plane lagsagainst a phase of linearly polarized light entering vertically into thefilm plane and having an oscillating surface of an electric vector in aY-Z plane. On the other hand, when uniaxially stretching at atemperature T2, a phase of linearly polarized light entering verticallyinto the film plane and having an oscillating surface of an electricvector in an X-Z plane leads against a phase of linearly polarized lightentering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane.

When a relation of Ts_(A)>Ts_(B) is satisfied, the temperature T1 ispreferably (Ts_(B)+3)° C. or more and (Ts_(A)+5° C. or less, and morepreferably (Ts_(B)+5° C. or more and (Ts_(A)+3° C. or less. Moreover,the temperature T2 is preferably (Ts_(B)+3° C. or less, and morepreferably Ts_(B) or less. The first stretching process is preferablyperformed at the temperature T1.

When a relation of Ts_(B)>Ts_(A) is satisfied, the temperature T2 ispreferably (Ts_(A)+3° C. or more and (Ts_(B)+5° C. or less, and morepreferably (Ts_(A)+5° C. or more and (Ts_(B)+3° C. or less. Moreover,the temperature T1 is preferably (Ts_(A)+3° C. or less, and morepreferably Ts_(A) or less. The first stretching process is preferablyperformed at the temperature T2.

The first stretching process may be performed by a conventionallywell-known method, for example, which includes a method of uniaxiallystretching in the longitudinal direction by using the difference of therim speed between rolls, a method of uniaxially stretching in thetransverse direction by using the tenter, and the like. The method ofuniaxially stretching in the longitudinal direction includes an IRheating method between rolls, a floating method, and the like. Thefloating method is suitable from the viewpoint of obtaining aretardation film with high optical uniformity. The method of uniaxiallystretching in the transverse direction includes a tenter method.

To reduce an irregular stretching and irregular thickness, it isallowable to make a difference of a temperature in a direction of awidth of the film at a stretching zone. To make the difference of thetemperature in a direction of the width of the film at the stretchingzone, well-known methods such as a method of adjusting a gate opening ofhot air nozzle in a direction of width, a method of setting IR heatersin a direction of width and controlling heating, and the like may beused.

(Second Stretching Process)

Then, the film is uniaxially stretched in a direction perpendicular tothe direction of the first uniaxial extension at a different temperatureT2 or T1 from the temperature in the above-mentioned first stretchingprocess. The second stretching process is preferably performed at atemperature T2 when a relation of Ts_(A)>Ts_(B) is satisfied, and ispreferably performed at temperature T1 when the relation ofTs_(B)>Ts_(A) is satisfied. In the second stretching process, a methodthat may be employed by the first stretching process may be employed asit is. A stretching ratio in the second stretching process is preferablysmaller than the stretching ratio in the first stretching process.

After the first stretching process and/or the second stretching process,a fixation process may be applied to the stretching film. A temperaturein the fixation process is usually (room temperature) to (the stretchingtemperature+30)° C., and preferably (the stretching temperature−40)° C.to (the stretching temperature+20)° C.

A method for producing a retardation film in the present invention caneasily give a retardation film having large area which satisfies arelation of 0.92≦R₄₀/Re≦1.08, in which Re is a retardation value at anincident angle of 0 degrees, and R₄₀ is a retardation value at anincident angle of 40 degrees; and/or a retardation film having largearea which satisfies a relation of n_(x)>n_(z)>n_(y), in which n_(x) isa refractive index in a slow axis direction in the plane, n_(y) is arefractive index in a direction perpendicular to the slow axis directionin the plane, and n_(z) is a refractive index in a thickness directionof the film.

The retardation film obtained by the producing method in the presentinvention is preferably 50 to 400 nm, and more preferably 100 to 350 nmin a retardation Re at an incident angle of 0 degrees in a wavelength of550 nm. Here, Re and R₄₀ are the values measured by using the parallelNicols rotation method (manufactured by Oji Scientific Instruments Co.,Ltd.; KOBRA-WR) in a wavelength of 550 nm. The refractive index n_(x),n_(z), and n_(y) are calculated by Re, R₄₀, film thickness, and averagerefractive index n_(ave) of the retardation film. The n_(ave) isdetermined according to the following expression:n _(ave)=Σ(n _(i) ×L _(i))/ΣL _(i)

n_(i): refractive index of an i-th layer resin

L_(i): film thickness of an i-th layer

The retardation film obtained by the producing method in the presentinvention may be preferably 0.5% or less, more preferably 0.3% or lessin a shrinkage percentage in a longitudinal and transverse directionafter heat-treating for 100 hours at 60° C. and 90% RH. When theshrinkage percentage exceeds this range, a transformation of theretardation film and flaking off from the display device are caused bythe shrinkage stress by using under the environment of the hightemperature and high humidity.

The retardation film obtained by the producing method in the presentinvention may be employed as single or in combination with othermaterials in a liquid crystal display device, an organic EL displaydevice, a plasma display device, a FED (field emission) display device,and a SED (surface electric field) display device, or the like, sincethe retardation film can give advanced birefringence compensations.

The liquid crystal display device comprises the liquid crystal panelthat a polarizing plate on an incident side of light, a liquid crystalcell, and a polarizing plate on an output side of light are arranged inthis order. A visibility of the liquid crystal display device may beimproved greatly by arranging the retardation film obtained by theproducing method in the present invention between the liquid crystalcell and the polarizing plate on the light incident side and/or betweenthe liquid crystal cell and the polarizing plate on the light outputside. A drive mode for the liquid crystal cell includes In PlaneSwitching (IPS) mode, Vertical Alignment (VA) mode, Multi domainVertical Alignment (MVA) mode, Continuous Pinwheel Alignment (CPA) mode,Hybrid Alignment Nematic (HAN) mode, Twisted Nematic (TN) mode, SuperTwisted Nematic (STN) mode, Optical Compensated Bend (OCB) mode and thelike.

The retardation film obtained by the producing method in the presentinvention may be stuck to the liquid crystal cell or the polarizingplate. The retardation film may be stuck to both sides of the polarizingplate or to one side of the polarizing plate. Also, two or more sheetsof the retardation films may be used. A well-known adhesive agent may beused for sticking.

The polarizing plate is composed of a polarizer and protective filmsstuck to the both sides of the polarizer. The retardation film may beused as a protective film by sticking the retardation film obtained bythe producing method in the present invention directly to the polarizerin place of the protective film. Since the protective film is omitted,the liquid crystal display device can be thinned.

EXAMPLES

The present invention will be explained more specifically with referenceto EXAMPLES in the following. However, the present invention, is notlimited to the examples. In the following EXAMPLES, “parts” or “%” is byweight unless otherwise specified.

(Thickness of Transparent Film)

A thickness of a film was measured by a contact thickness gauge.

A film was embedded into an epoxy resin, the film was sliced into piecesusing a microtome (manufactured by YAMATO KOHKI INDUSTRIAL Co., Ltd.;“RUB-2100”), and then the cross-section of the piece was observed usinga scanning electron microscope to determine a thickness of the eachlayer composing the film.

(Light Transmittance)

Alight transmittance of a film was measured by using a spectrophotometer(manufactured by JASCO Corporation; Ultraviolet Visible Near-infraredSpectrophotometer “V-570”) in accordance with JIS K 0115.

(Deflection Temperature Under Load)

A deflection temperature under load of the resin was measured by using atest piece made in accordance with JIS K 7191.

(Re, R₄₀, Angle of Slow Axis)

Re, R₄₀, an angle of the slow axis to a longitudinal direction and ofthe film in a wavelength of 590 nm were measured by using the parallelNicols rotation method (manufactured by Oji Scientific Instruments Co.,Ltd.; KOBRA-WR).

The similar measurement was performed in a direction of width of thephase difference film at equal intervals by ten points, and the meanvalue was calculated.

Moreover, average refractive index n_(ave) of the laminated film wasdetermined according to the following expression.n _(ave)=Σ(n _(i) ×L _(i))/ΣL _(i)

n_(i): refractive index of i-th layer resin

L_(i): film thickness of i-th layer.

In addition, n_(x), n_(y), and n_(z) of the laminated film werecalculated from the above-mentioned Re, R₄₀, n_(ave), thickness and ofthe film.

Producing Example 1

A film molding device for coextrusion molding of two kinds and twolayers was prepared. Then, pellets of polycarbonate resin (Made by AsahiChemical Industrial Co., Ltd., WONDER LIGHT PC-110, a deflectiontemperature under load is 145° C.) were put in one uniaxially extrudingmachine which was equipped with a screw of the double flight type, andwere melted.

Pellets of the styrene-maleic anhydride copolymer resin (made by NOVAChemicals Ltd., Dylark D332, a deflection temperature under load is 135°C.) were put in the other uniaxially extruding machine which wasequipped with a screw of the double flight type, and were melted.

The melted polycarbonate resin at a temperature of 260° C. was suppliedto one manifold in a multi manifold die (surface-roughness of the dielip Ra is 0.1 μm) through a polymer filter having a leaf disk shape anda mesh spacing of 10 μm, and the melted styrene-maleic anhydridecopolymer resin at a temperature of 260° C. was supplied to the othermanifold in the multi manifold die through a polymer filter having aleaf disk shape and a mesh spacing of 10 μm.

The polycarbonate resin and the styrene-maleic anhydride copolymer resinwere extruded from the multi manifold die at the same time at 260° C.and were made into a melted resin film.

The melted resin film was casted on a cooling roll modulated at 130° C.in the surface temperature, and then was passed between two coolingrolls that was modulated at 50° C. in the surface temperature to obtaina laminated film 1 being 1350 mm in width and 180 μm in thicknesscomposed of the polycarbonate resin layer (layer A: 20 μm) and thestyrene-maleic anhydride copolymer resin layer (layer B: 160 μm).

Producing Example 2

A laminated film 2 being 1350 mm in width and 180 μm in thicknesscomposed of the polycarbonate resin layer (layer A: 36 μm) and thestyrene-maleic anhydride copolymer resin layer (layer B: 144 μm) wasobtained in the same manner as in PRODUCING EXAMPLE 1, except that thethickness of layer A was adjusted to 36 μm and the thickness of layer Bwas adjusted to 144 μm.

Producing Example 3

A laminated film 3 being 1350 mm in width and 180 μm in thicknesscomposed of the polycarbonate resin layer (layer A: 10 μm) and thestyrene-maleic anhydride copolymer resin layer (layer B: 170 μm) wasobtained in the same manner as in PRODUCING EXAMPLE 1, except that thethickness of layer A was adjusted to 10 μm and the thickness of layer Bwas adjusted to 170 μm.

Producing Example 4

A laminated film 4 being 1350 mm in width and 180 μm in thicknesscomposed of the polycarbonate resin layer (layer A: 80 μm) and apolystyrene resin layer (layer B: 90 μm) was obtained in the same manneras in PRODUCING EXAMPLE 1, except that the polystyrene resin (made byJapan Polystyrene Inc., HF44, a deflection temperature under load is 73°C.) was used in place of Dylark D332, the thickness of layer A wasadjusted to 80 μm and the thickness of layer B was adjusted to 90 μm.

The laminated films 1 to 4 were uniaxially stretched by 1.25 times ofstretching ratio in various temperatures in a longitudinal direction ofthe film. Table 1 shows the lag in a phase of which linearly polarizedlight entering vertically into the film plane and having an oscillatingsurface of an electric vector in an X-Z plane against linearly polarizedlight entering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane, in which an X axis is anuniaxially stretching direction, a Y axis is a direction perpendicularto the uniaxially stretching direction in the film plane, and a Z axisis a direction of a thickness of the film.

It is understood that the phase of linearly polarized light enteringvertically into the film plane and having an oscillating surface of anelectric vector in an X-Z plane against the phase of linearly polarizedlight entering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane in the laminated film 1,lags at 140° C. and 145° C. (a sign of retardation in phase is plus) andleads at 130° C. (a sign of retardation in phase is minus). It isunderstood that the phase of other laminated films didn't reversebetween 120° C. to 145° C.

[Tab. 1]

TABLE 1 Retardation in phase at each stretching temperature (nm) 120° C.130° C. 135° C. 140° C. 145° C. 150° C. Laminated Break- −205.1 10.997.9 88.6 42.9 film 1 ing Laminated Break- 16.6 228.3 279.0 233.8 137.2film 2 ing Laminated Break- −343.7 −125.0 −15.4 −2.2 −16.1 film 3 ingLaminated 1913.5 1832.2 1734.6 1682.8 1359.4 867.9 film 4

Example 1

The laminated film 1 obtained in PRODUCING EXAMPLE 1 was supplied to alongitudinal uniaxially stretching machine, and the film was stretchedin a longitudinal direction at a stretching temperature of 145° C. by astretching magnification of 1.5 times.

Then, the stretched film was supplied to the tenter stretching machine,the film was stretched in a transverse direction at a stretchingtemperature of 130° C. by a stretching magnification of 1.25 times, anda retardation film 1 was obtained. The evaluation results are shown inTable 2.

Example 2

A retardation film 2 was obtained in the same manner as in EXAMPLE 1,except that the transversely stretching magnification was changed to 1.5times. The evaluation results are shown in Table 2.

Comparative Example 1

A retardation film 3 was obtained in the same manner as in EXAMPLE 1,except that the transversely stretching temperature was changed to 145°C. The evaluation results are shown in Table 2.

Comparative Example 2

A retardation film 4 was obtained in the same manner as in EXAMPLE 1,except that the laminated film 1 was changed to the laminated film 2.The evaluation results are shown in Table 2.

Comparative Example 3

A retardation film 5 was obtained in the same manner as in EXAMPLE 1,except that the laminated film 1 was changed to the laminated film 3.The evaluation results are shown in Table 2.

Comparative Example 4

A retardation film 6 was obtained in the same manner as in EXAMPLE 1,except that the laminated film 1 was changed to the laminated film 4 andthe transversely stretching temperature was changed to 70° C. The filmafter a transversely stretching whitened a little. The evaluationresults are shown in Table 2.

[Tab. 2]

TABLE 2 Example Comparative Example 1 2 1 2 3 4 Unstretched 1 1 1 2 3 4film Longitudinal 145 145 145 145 145 145 direction stretchingtemperature (° C.) Longitudinal 1.5 1.5 1.5 1.5 1.5 1.5 directionstretching magnification Transverse 130 130 145 130 130 70 directionstretching temperature (° C.) Transverse 1.25 1.5 1.25 1.25 1.25 1.25direction stretching magnification n_(x) 1.5379 1.5825 1.5817 1.54391.5814 1.5887 n_(y) 1.5354 1.5795 1.5811 1.5411 1.5790 1.5864 n_(z)1.5367 1.5813 1.5805 1.5410 1.5814 1.5799 Re 247.8 238.5 59.8 270.2233.7 224.0 R₄₀ 247.5 235.4 75.7 299.2 216.1 357.0 R₄₀/Re 1.00 0.99 1.271.11 0.93 1.59

As mentioned above, a retardation film having a large area whichsatisfies a relation of n_(x)>n_(z)>n_(y), in which n_(x) is arefractive index in a direction of a slow axis in a plane of the film,n_(y) is a refractive index in a direction perpendicular to thedirection of the slow axis in the plane, and n_(z) is a refractive indexin a thickness direction; or satisfies a relation of 0.92≦R₄₀/Re≦1.08,in which Re is a retardation value at an incident angle of 0 degrees,and R₄₀ is a retardation value at an incident angle of 40 degrees, canbe highly accurate and easily manufactured by uniaxially stretching anoriginal film for producing retardation film in one direction at eithera temperature T1 or T2, and then, uniaxially stretching in a directionperpendicular to the above-mentioned direction of stretching at adifferent temperature T2 or T1 from the above-mentioned temperature, inwhich the original film for producing the retardation film has acharacteristic that a phase of linearly polarized light enteringvertically into a film plane and having an oscillating surface of anelectric vector in an X-Z plane against a phase of linearly polarizedlight entering vertically into the film plane and having an oscillatingsurface of an electric vector in a Y-Z plane lags by uniaxiallystretching in a direction of X axis at a temperature T1, and leads byuniaxially stretching in a direction of X axis at a differenttemperature T2 from the above-mentioned temperature T1, in which X axisis the uniaxially stretching direction, Y axis is a directionperpendicular to the uniaxially stretching direction in the film plane,and Z axis is a direction of a thickness of the film.

What is claimed is:
 1. A method for producing a retardation film which satisfies a relation of n_(x)>n_(z)>n_(y), in which n_(x) is a refractive index in a direction of a slow axis in a plane of the film, n_(y) is a refractive index in a direction perpendicular to the direction of the slow axis in the plane, and n_(z) is a refractive index in a thickness direction, the method comprising the steps of: (a) uniaxially stretching an original film for producing retardation film in one direction at a temperature T1 which is not less than (Ts_(B)+5)° C. and not more than (Ts_(A)+3)° C.; and then (b) uniaxially stretching the film stretched in the step (a) in a direction perpendicular to the direction of stretching in the step (a) at a temperature T2 which is not more than (Ts_(A)+3)° C., wherein the original film for producing the retardation film is a laminate comprising a layer comprising a thermoplastic resin A having a positive intrinsic birefringence and a deflection temperature under load (Ts_(A)), and a layer comprising a thermoplastic resin B having a negative intrinsic birefringence and a deflection temperature under load (Ts_(B)), the deflection temperature under load (Ts_(A)) is higher than the deflection load temperature under (Ts_(B)), and wherein if and when the original film is stretched at the temperature T2, an absolute value of a phase difference which appears in the resin A is small than an absolute value of the phase difference which appears in the resin B, and if and when the original film is stretched at the temperature T1, an absolute value of the phase difference which appears in the resin B is smaller than an absolute value of the phase difference which appears in the resin A.
 2. The method according to claim 1, in which an absolute value of difference between the deflection temperature under load Ts_(A) of the thermoplastic resin A and the deflection temperature under load Ts_(B) of the thermoplastic resin B is 5° C. or more.
 3. The method according to claim 1, in which a rupture elongation of the thermoplastic resin A at the temperature TS_(B) and a rupture elongation of the thermoplastic resin B at the temperature Ts_(A) are both 50% or more.
 4. The method according to claim 1, in which the thermoplastic resin A is a polycarbonate resins and the thermoplastic resin B is a polystyrene resins.
 5. The method according to claim 1, in which a ratio of sum total thickness of layers composed of the thermoplastic resin A and sum total thickness of layers composed of the thermoplastic resin B is 1:5 to 1:10. 